Hyperkalemia is estimated to occur in 21% of hospitalized patients. However, the mechanism for eliminating high quantities of K during low Na intake is not known. Conventionally, it was thought to occur by direct exchange of K for Na in principal cells of the distal nephron, but when plasma [K] is high with little Na intake, K secretion must be uncoupled from Na reabsorption. Without an understanding of the K channels that mediate this process, better therapies for victims with acutely elevated plasma [K] cannot be developed. Our long-term goal is to understand the role of specific renal ion channels to eliminate K. Our focus is the large, Ca-activated K channels (BK) in the distal nephron that contain the accessory BK-b4 (BK-a/b4) in base-secreting (IC-b) and acid-secreting (IC-a) intercalated cells. The objective here, which is a critical step toward our long-goal, is to examine the role of BK-a/b4 in secreting K under high K conditions. We reported that when BK-b4 knock-out mice (b4-KO) are placed on a high K diet for 7-10 days (K adapted;KA), they exhibit increased plasma [K] and a 40% reduction in the capacity to eliminate K compare to wild type. Furthermore, high flow associated with a high K diet activates BK-a/b4, causing a release of K and cell size reduction. Thus, BK-a/b4 play a major role in how the kidney handles a high K diet;however, the mechanism by which BK-a/b4 function in this capacity is not well understood. The central hypothesis is that the elevated aldosterone of KA mice enhances K secretion by a Na-independent mechanism that utilizes K secretion by BK-a/b4 in the IC-b and K recycling by BK-a/b4 in the IC-a of the outer medullary collecting duct (OMCD). This hypothesis will be tested by pursuing three specific aims: 1. Determine whether aldosterone or high plasma K enhances BK-a/b4-mediated K secretion in KA mice. 2. Determine the role of BK-a/b4 in Na-independent K secretion. 3. Determine the roles of BK-a/b4 and KCC4 in the OMCD K recycling and flow enhancement. For these studies, we will employ methods including renal clearance measurements, patch clamping and immunohistochemical analysis of WT and b4-KO mice. The approach is innovative because we test a new model of Na-independent K secretion while isolating each potential mechanism. The proposed research is significant because it is expected to answer some long- standing questions regarding the mechanism for eliminating high quantities of K during very small amounts of Na intake. Such understanding may help prevent the large number of deaths occurring each year from treating heart failure subjects with aldosterone receptor blockers that lead to hyperkalemia-induced ventricular arrhythmias. PUBLIC HEALTH RELEVANCE: Many elderly and heart failure patients suffer fatal heart attacks because of medications that increase the potassium in the blood. These proposed studies are designed to determine how a recently discovered protein in kidney cells, called a BK-a/b4 channel, eliminates the potassium when blood potassium is elevated. The proposed research is relevant to the mission of the NIH to develop fundamental knowledge that will help to design strategies to intervene in high blood potassium-induced abnormalities in heart function.